1. Field of the Invention
The present invention concerns a turbocharger of the type set forth in the precharacterizing portion of claim 1.
2. Description of the Related Art
In turbomachinery in which the turbine is intended to drive a compressor or the like, it is often desirable to control the flow of motive gas to the turbine to improve its efficiency or operational range. In order to accomplish this, the nozzle passages leading to the turbine wheel may be of variable geometry. These variable geometry nozzle passages can be provided by means of a plurality of blades which are pivotable so as to alter the configuration of the passages therebetween. The design of the suspension system used in association with the pivoting blade design is critical to prevent binding of either the suspension system or the blades.
U.S. Pat. Nos. 2,860,827 and 4,179,247 disclose designs to prevent binding of the pivoting blade actuation mechanism. However, none of the above-mentioned designs are suspension systems for an actuation system which accommodates the thermal cycling experienced by the turbine housing as well as the components of the actuation system.
U.S. Pat. No. 2,860,827, the content of which is to be considered as incorporated by reference herein, describes a turbocharger with variable turbine geometry. Exhaust gasses flow radially past nozzles in a ring-shaped passage situated between the housing part of the turbine housing and the nozzle ring, which nozzles are defined by the intermediate space between nozzle blades which are arranged in a ring and mounted pivotably along the nozzle ring, and operate in such a manner that the nozzles are maximally opened when the nozzle blades are radial, and maximally closed when the nozzle blades are essentially oriented tangential.
The nozzle blades are mounted to the nozzle ring by means of pins, which extend through the nozzle ring, and which carry actuating arms on their opposite ends.
Provided on the same plane as the circularly arranged actuating arms there is a second ring, the so-called actuating ring, for simultaneous actuation of all actuating arms, for which the actuating ring on its inner edge includes engagement means which cooperate with corresponding engagement means on each of the actuating arms, so that with limited coaxial pivoting of the actuating ring with respect to the nozzle ring all actuating arms, and the therewith associated nozzle blades, can be pivoted.
Associated with the actuating ring is an actuating means which extends through the turbine housing in order to control the actuating ring from outside the housing.
The actuating ring is carried by a certain number of rollers each of which is provided with a groove, and guided for limited rotation, which rollers can be arranged in a circular pattern corresponding to the inner edge of the actuating ring.
These rollers can rotate freely about pins, which pins can be provided in the same part of the turbine housing as the above described actuating means.
The pins extend through the wall of the turbine housing and are axially fixed directly outside and inside this wall freely rotatably by means of spring rings.
This arrangement functions in satisfactory manner as long as no great temperature oscillations occur.
Turbochargers are however subjected to very strong temperature oscillations as a result of the flowing through of hot exhaust gasses through the turbine part, so that the turbine part and adjacent parts are heated up to 900° C.
These frequently extereme temperature oscillations, together with the extremely high rotation speed of the turbine wheel and the compressor wheel, produce extreme stresses for all components, which results in an early expenditure and loss of function of the turbocharger.
It is particularly important that the geometric configuration of all cooperating parts, such as nozzle ring, actuating ring, rollers and pins, etc. remain intact, without thermal misalignment and hysteresis.
The turbocharger according to U.S. Pat. No. 2,860,827 is not optimally designed in order to maintain the geometry of the described parts in the case of large thermal oscillations.
U.S. Pat. No. 4,179,247 describes a turbocharger and in particular a suspension mechanism for the actuating ring, which is in the form of a double ball bearing.
This type of ball bearing is particularly critical in the above-mentioned conditions and is beyond this very complicated in its construction.
Many attempts have been made in order to solve the above described problem, and in part these problems were solved by the turbocharger according to European Patent 0 226 444 (U.S. Pat. No. 4,804,316).
This patent describes a suspension mechanism for the actuating ring with pins and rollers, the rollers having circumferential grooves, which can carry and guide the actuating ring in a manner similar to that of U.S. Pat. No. 2,860,827.
In EP-0226444 the roller pins are however not fixed axially in the housing, but rather they extend freely between bores in the housing on one side to bores in the nozzle ring on the other side, wherein a certain separation is maintained between the inner side of the housing and the opposing side of the nozzle ring in order to produce a second ring gap, and wherein the grooved rollers are provided for free rotation on the pins within this second ring gap.
Since the ends of the pins engage in the nozzle ring, the effect is to provide an exact co-axiality of nozzle ring and actuating ring.
In practice however two problems are associated therewith.
On the one hand the construction of the actuating mechanism according to U.S. Pat. No. 2,860,827 is complicated, and the introduction of the roller pins in the bores first in the nozzle ring or the housing, thereafter the seating of the rollers upon the pins and the actuating ring upon the rollers and then the introduction of the free ends of the pins into the bores in the respective other element (housing or nozzle ring) element is very difficult to accomplish without an exact, axially parallel arrangement of these free ends of the pins.
This is a true test of finesse, since the orientation of the bores between the two elements is never perfect, and besides this, because of the necessary tolerance between pin and bore, the pins tend to be tilted or askew prior to introduction into the second element.
On the other hand the bores in the housing and those in the nozzle ring are subjected to different thermal dilations, so that in operation the pins are directed away from their exact axially parallel orientation, which detracts from the friction free operation of the parts.